In my prior U.S. Pat. No. 3,963,466, there was provided a unitary vessel for removing water vapor from gases and particularly for removing water vapor from compressed air. Prior to my previous invention, the functions of cooling, drying, and warming were normally performed in separate distinct vessels requiring substantial ducting and utilizing considerable space. In accordance with the invention of my aforesaid patent, all of those functions were condensed into a unitary vessel with resultant savings in space, energy, and cost. The apparatus shown in said earlier patent, however, found its most efficient application in situations where relatively large quantities of compressed air were being dealt with, with the unit considered to be somewhat less efficient on a cost basis in situations where a relatively small quantity of compressed air was to be dried. It is one of the purposes of the present invention to overcome the situation just mentioned.